1. Field of the Invention
The present invention relates to digital to analog conversion technology, and specifically to a sigma-delta digital to analog converter having dynamic spectral shaping.
2. Description of Related Art
Communications components such as CDMA processing transmitters require that a processed digital signal be converted back to an analog signal through a digital to analog converter circuit (DAC circuit) prior to transmission. A sigma-delta DAC circuit is preferred in such applications because it is simpler in design than a conventional DAC circuit. This is because a higher percentage of the design of a sigma-delta DAC circuit is in the digital domain compared to a conventional DAC circuit, which is implemented with a larger number of analog components such as resistors, transistors and current sources. In addition, because such a large portion of its design is in the digital domain, a sigma-delta DAC circuit can generically be re-used in subsequent higher frequency applications and processes, while a conventional DAC circuit is typically tied to a particular application due to its relatively large number of analog components and therefore must be re-engineered prior to use in each subsequent application.
While sigma-delta DAC circuits are advantageous with respect to conventional DAC circuits in many applications, certain limitations do exist with respect to their design and operation. For example, a conventional sigma-delta modulator with static characteristics may require a higher order filter, such as a 3rd or 4th order filter, or higher operating frequency to meet similar specifications. These higher order filters require more operating current, and are less stable than lower order filters. Further, DACs used in conventional sigma-delta DAC circuits are susceptible to power supply noise because of their single input, single output design. The above limitations of a sigma-delta DAC circuit must therefore be considered in the design of, for example, wideband CDMA communications devices in which parameters such as cost and battery power are critical.
What is needed is a sigma-delta DAC circuit with dynamic spectral shaping that provides results similar to higher order sigma-delta DAC circuits at lower associated costs and power requirements.